Drive shafts of composite construction are well known. Composite drive shafts find particular utility in those applications requiring a strong but lightweight member to transmit torque between spaced apart couplings. Such shafts are particularly desirable desirable in modern aircraft and automobile power train applications.
A conventional composite drive shaft is of tubular construction and generally includes a series of layers of parallel filaments bonded together in a matrix and disposed at a predetermined lay angle with respect to the longitudinal axis of the shaft. The shaft is formed by coating the filaments with a bonding agent, wrapping the same about a mandrel at the desired lay angle and subsequently removing the mandrel. The lay angle is of substantially the same magnitude in each layer but is of opposite direction so that the filaments in adjacent layers cross one another. Preferably, the filaments are of so-called high strength materials such as glass, carbon, polyamide, and the like. The matrix material binding the filaments together may be of a relatively stiff material, such as epoxy, or of a more flexible material, such as polyurethane.
In designing composite shafts, the designer is faced with certain trade-offs. For instance, it is known that torsional stiffness is maximized when the filament lay angle is relatively large, but in such event, the bending stiffness is relatively low. At relatively small lay angles, the bending stiffness increases, but the torsional stiffness decreases. To accommodate these considerations, it has been conventional practice to construct composite shafts having filament lay angles of 45 degrees. One or more other factors can then be varied to achieve the desired design goals, such as varying the dimensions of the shaft, the stiffness of the filaments or the matrix material.
Certain problems are encountered in designing a satisfactory composite shaft capable of meeting the requirements of both transmitting substantial torque and accommodating the bending deflections caused by misaligned end couplings. For instance, such a shaft should have a relatively high torsional stiffness to bending stiffness ratio but should have minimal bending stresses in order to accommodate size and weight considerations. Attempts to solve this problem by utilizing so-called low modulus elastomeric bonding agents, such as polyurethane, with filaments disposed at conventional 45 degree lay angles provide the desired stiffness ratio; however, the bending stress in the resulting shaft structure is significantly higher than desired, thereby reducing the torque transmission capacity of a shaft of a given size and weight. Accordingly, there is a need for a filament wound composite shaft which is torsionally stiff yet sufficiently flexible to accommodate endwise misalignments while being light in weight and durable.